The plan ahead now is to buy a few replacement roms (modern variants with identical pin-out) and copy the images over and see if they work. Once they work I can start modifying the code slightly.

Figuring out what to change in the code and where might seem harder than it is. I thought about ho to do it and I came up with the idea that the strings tell me something about what the code does, so all I have to do is to follow the strings and see which code pushes the strings around. Next I just identify which strings relate to which functions (such as loading/saving MSD data) and hijack those routines by jumping to an unused location where I have some space for my own code.